Magma mixing revealed from in situ zircon U–Pb–Hf isotope analysis of the Muhuguan granitoid pluton, eastern Qinling Orogen, China: implications for late Mesozoic tectonic evolution
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Felsic
Xenolith
Igneous differentiation
Hornblende
Electron probe microanalyses of biotite and hornblende are presented for a variety of granitoids from Peninsular Malaysia and Sumatra. Most biotites from the Main Range have Fe/(Fe+Mn+Mg) mole ratios greater. than 0.62, with exceptions at Sg. Sok and the Cameron Highlands, which have !-type characteristics. The Eastern Belt granitoids commonly contain hornblende and biotite. The biotite shows a wide range ofFe/(Fe+Mn+Mg), with higher values from the known tin mineral ized areas of Jemaluang and Kuantan. Hornblendes and co-existing biotites generally exhibit equilibrium partitioning of Fe/Mg proving a magmatic origin for both. The total aluminium in the amphiboles prove an epizonal environment of emplacement for the Eastern Belt granitoids and an even higher sub-volcanic environment for theSE province. The known occurrences of amphibole in the Main Range are few (Sg. Sok in Kedah and Bk. Berapit in Perak). They are of actinolitic hornblende and exhibit a non equilibrium partitioning of Fe/Mg with the biotite. They are probably of hydrothermal origin. An unusually green coexisting biotite and hornblende in the Gunung Benom area of the Central Belt appears to represent a deeper level of emplacement than the Main Range or Eastern Belt. The biotite-hornblende assemblage is not in equilibrium and the biotite is unusually enriched in magnesium. A reconnaissance overview of the chemical variation of biotites and hom blendes of Malaysian and Sumatran granitoids is presented. The samples analysed in the present study are listed in Table 1. The majority of the samples are from Peninsular Malaysia, with three granitoids from Sumatra and one trondhjemite from the gabbro layer of the Sabah ophiolite included for compari son. Brief reference is also made to feldspar, chlorite and muscovite analysis made during the course of the study. All analyses were carried out on carbon coated polished thin sections in an electron probe microanalyser using a computer-controlled energy dispersive spectrometer for all elements except fluorine, for which a wavelength spectrometer was us.ed. The preliminary results of this study were presented at the GEOSEA IV Conference in Manila in November 1981, but were not compiled for publication in the proceedings. Liew (1983) carried out a considerable number of microprobe analyses of such minerals as feldspar, biotite, muscovite, tourmaline, rare garnet, hom-
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The Tabito composite mass in the Abukuma Mountains was dated for its biotite and hornblende separates by 40Ar-39Ar and K-Ar methods. K-Ar biotite ages ranging from 111 to 95 Ma are significantly older than the coexisting hornblendes in two of four rock samples examined. 40Ar-39Ar age spectra of biotite and hornblende show argon-loss and convex-upward patterns. However, higher temperature fractions of hornblende yielded well defined plateau ages of 103.1±1.4 Ma for the Iritabyuto granodiorite and 101.7±1.2 Ma for Myojin-ishi tonalite, which are consistent with a conventional K-Ar age of hornblende from the Komuro tonalite by Shibata and Uchiumi (1983). 40Ar-39Ar plateau ages of hornblende support the cooling rate (∼15°C/m.y.) of the Tabito mass by Tanaka et al. (1999).
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The present study aims at understanding how humic acids (HAs) (extracted from Entisol, Alfisol and Mollisol) affect and transform primary silicate minerals, viz., hornblende (inosilicate) and biotite (phyllosilicate), and aid in nutrient release therefrom. The focus is on the kinetics of dissolution and the alterations brought about in the products formed after treatment with HA. The kinetic curves exhibit alternate crests and troughs pointing to alternate solubilisation and precipitation. Structural difference between the two primary silicates lead to differential release of cations. The contrast between the minerals is more pronounced when the altered products are compared. While hornblende shows drastic changes including reduction in Fe2+/3+ and Ca2+ and enrichment in Si4+, Al3+ and Mg2+, biotite is almost minimally altered. XRD(X ray diffraction) studies indicate the disappearance/attenuation of many prominent bands of hornblende in the weathered residues with several new crystalline phases. Biotite, however, is only slightly altered with the formation of a few new phases. IR studies suggest deposition of HA on the surfaces of both biotite and hornblende but such deposition is far more prominent with hornblende.
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We report adventages of employing MgO as a differentiation index for the Namwon granitic complex. It is shown to be much more sensitive than the usual Harker index. The complex can be divided into two groups on the basis of /MgO ratio. The low /MgO group consists of hornblende biotite tonalite-granodiorite, porphyritic hornblende biotite granodiorite (PHBGd) and part of biotite granite (loBG). PHBGd shows its own distinct variation in the low group. This group is characterized in most cases by the presence of hornblende, even if it occurs as a trace amount. The high /MgO group consists of part of biotite granite (hiBG) and two mica granite. The major element differences between rock types are also apparent in biotite chemistry. These chemical data indicate that at least two distinct origins of magma are rquired for the complex. Two kinds of biotite granite revealed in this study show distinct geographic distribution, suggesting that a new geologic map should be made.
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Igneous differentiation
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Biotite and hornblende from the western portion of the Cortlandt complex record undisturbed $$^{40}Ar/^{39}Ar$$ incremental-release spectra with average total-gas dates of 420 m.y. (hornblende) and 390 m.y. (biotite). Biotite from the younger phase of the Rosetown pluton also records undisturbed release spectra with an average total-gas date of 420 m.y. Coexisting hornblendes display more complex spectra (a result of excess argon contamination); however, they have similar high-temperature "plateaus" at 450 m.y. Both biotite and hornblende from the older portion of the Rosetown have disturbed release spectra, possibly a result of excess argon contamination (hornblende) and partial postcrystallization gas loss (biotite). The disturbed spectra preclude a definitive estimate of absolute age but the data do suggest a minimum date of ~485-500 m.y. Because field criteria (Ratcliffe 1968, 1971) clearly indicate that both the Cortlandt and the younger Rosetown postdate a regionally pervasive progressive metamorphism, it is suggested that their differences in geochronology reflect regional variations in times of postmetamorphic cooling and that the 450-m.y. date defined by the Rosetown hornblendes closely approximates the time of intrusion of both plutons. This conclusion requires that the regional metamorphic event was Taconic in age. The undisturbed incremental release spectra of the Cortlandt samples indicate no substantial argon loss which could be attributed to Acadian reheating. This substantiates the conclusion that regional metamorphic zonation is Taconic rather than Acadian, and suggests that this area was little affected by Devonian or later reheating. The difference in probable age of the older part of the Rosetown (485-500 m.y.) and the cross-cutting younger phase (450 m.y.) bracket a period of reactivation of the Ramapo fault system and suggests that fault activity was concurrent with Middle Ordovician Taconic metamorphism.
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Several specimens of coexisting biotite and hornblende were separated from the rapakivi granites, Svecofennian and Karelian granitic rocks of Finland and subjected to D/H ratio measurements and chemical analyses. D/H distribution between biotite and hornblende in the rapakivi and Svecofennian granitic rocks showed the disequilibrium situation defined by KURODA et al. (1977a). The origin of equilibrium and disequilibrium granites was discussed on the basis of the content and state of water in magmas.
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1974)
The deuterium content (δDSMOW) of the water extracted from coexisting biotite and hornblende in granitic rocks was measured. The relationship between δD of biotite and hornblende suggested an isotopic equilibrium in the rocks of four granitic bodies in the Kitakami mountains (Northeast Japan), and the maximum temperature of isotopic equilibration of biotite and hornblende was estimated to be 700-1, 000°C, provided that the minimum temperature was 450-550°C. The δD value obtained for the water with which these minerals equilibrated was -29_??_-37‰ and is definitely enriched in deuterium relative to the present local surface water. On the other hand, the relationship between δD of biotite and hornblende in the Ryoke metamorphic belt contiguous to the Median Dislocation Line (Southwest Japan) indicated an isotopic disequilibrium. In this case a complicated history of formation of these granitic rocks in terms of the interaction of water and hydrous silicates was inferred.
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40Ar/39Ar analyses on hornblende and biotite samples separated from the Tanzawa tonalite demonstrate that most samples have highly disturbed K-Ar systematics. Expect for Mt. Ishiwari samples, unrealistically old ages are commonly observed in lower temperature fractions. Especially, Omatazawa samples showed severely disturbed age spectra, which strongly suggests that one should not use the previously reported 10 Ma hornblende K-Ar ages in the southwestern margin of the Tanzawa pluton in a discussion of the cooling history of the pluton. In the eastern part, although showing a clear evidence of excess argon in the lower temperature fractions, the biotite and hornblende in the Yushin sample yield poorly defined high temperature plateaus about 5 Ma. On the other hand, the biotite and hornblende separated from the Mt. Ishiwari sample reveal apparently good plateaus in age spectra. However, the biotite has a plateau age older than that for the cogenetic hornblende, which is consistent with our previous K-Ar results on these samples, but is inconsistent with the widely accepted closure temperature hypothesis. Comparing with our previous K-Ar results, the 6.26 +/- 0.17 plateau age for the hornblende could be interpreted as the time when the sample cooled to the closure temperature of hornblende (about 500°C). The older age of the biotite should be due to excess argon.
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